This invention generally relates to methods and apparatus for welding plastic parts using ultrasonic wave energy. In particular, the invention relates to methods and apparatus for pre-sealing or stomping two interlocked zipper strips at spaced intervals along the zipper.
Reclosable bags are finding ever-growing acceptance as primary packaging, particularly as packaging for foodstuffs such as cereal, fresh vegetables, snacks and the like. Such bags provide the consumer with the ability to readily store, in a closed, if not sealed, package any unused portion of the packaged product even after the package is initially opened.
Reclosable plastic zipper assemblies are useful for sealing thermoplastic pouches or bags. Typically, the plastic zippers include a pair of interlockable profiled members that form a closure. The profiles in plastic zippers can take on various configurations, e.g. interlocking rib and groove elements having so-called male and female profiles, interlocking alternating hook-shaped closure elements, etc.
Machinery for continuously manufacturing zippered bags at high speed is well known. A typical pre-made bag system comprises a station for continuously paying out a web of bag making film from a roll; a folding mechanism for folding the film; a station for continuously paying out zipper and inserting the zipper between the opposing walls of the folded bag making film in the machine direction; a station for continuously sealing (e.g., by conducting heat) the zipper to both walls of the bag making film; a station for ultrasonically welding the zipper at regular package width intervals; a station for cross sealing (again using heat conduction) the zipper/film assembly at the same regular package width intervals to form side seals; and a station for cutting the zipper/film assembly along a midline of the zipper crush and the cross seal to sever a bag from the remainder of the film. In a known pre-made bag system, the film moves continuously through the zipper sealing station, but intermittently through the ultrasonic welding, cross sealing and cutting stations. This transition is effectuated by a synchronization section having motor-driven pinch rolls and a dancer assembly that converts continuous motion into start-stop motion.
Known pre-made bag systems of the above-described type can produce bags at a rate of 200 bags per minute. It would be desirable to increase the production rate of such pre-made bag systems. The limiting factor in speed is the time required to perform an ultrasonic crush of the zipper.
An apparatus for high-speed ultrasonic welding of zipper strips was disclosed in U.S. patent application Ser. No. 10/216,252 filed on Aug. 8, 2002 and entitled “Sealer with Continuous Motion Ultrasonic Zipper Welding”. However, there is a continuing need for improvements in apparatus for ultrasonically welding zipper strips at high speeds.